This invention relates to lead frames for the manufacture of semiconductor devices encapsulated in a plastic material and more particularly to lead frame structures which are adapted to providing semiconductor devices having an electrically isolated heat sink while still using a multiple unit preassembly structure which is amenable to reduced cost automated manufacturing techniques.
The development of more highly automated techniques for the manufacture of semiconductor devices has resulted in the development of various types of lead frames by which the relatively small and fragile metallic device leads ultimately associated with a semiconductor device are initially formed as a multiple unit preassembly structure which is arranged to maintain mechanical rigidity by use of tie bars and which is adapted for automatic assembly techniques with indexing holes. These multiple unit lead frames allow automatically indexed sequential operations and batch handling and result in greatly reduced manufacturing costs. Initially these lead frame approaches were proposed for low power transistor structures which were relatively small as in Helda et al, U.S. Pat. No. 3,444,441. Later developments expanded these techniques to physically larger transistor structures having much higher power dissipation requirements as in Seigerson, U.S. Pat. No. 3,574,815. When semiconductor structures having higher power dissipation requirements are manufactured, an additional constraint is introduced in that the portion of the lead frame structure used to provide physical support for the semiconductor device must now be adapted to not only physically hold the semiconductor device but to effectively conduct heat away from this semiconductor device. This thermal conduction avoids the buildup of high temperatures within the semiconductor device junctions which tend to destroy the device or reduce its long term reliability. In many applications, it is a further requirement that the semiconductor device include a heat sink metallic portion for mounting the semiconductor chip which is electrically isolated from the leads of the semiconductor device. Also, the recent development of high power dissipation integrated circuits such as drivers, regulators and power amplifiers have resulted in a requirement for low cost plastic packages with more than three device leads which have a high power dissipation capability.